So i have not winterized my 2004 Sky Supreme V 230 with a Mercruiser MPI 350. It's stored in a Breezeway type thing at my house here in Santa Rosa CA. I'm leaving for Tahoe Tomorrow. should i be worried. Plus What should i do with it when I get back.

You havent given us near enough info to tell you how worried you should be. does it get cold in there? if you left a bottle of water there over night will it freeze? The fact that you're asking " should I be worried" probably means you should be worried... I had no idea where that was so I did a little research and it looks like you're in for a freeze tonight...
Considering the engine block makes it then the first thing I would do is change the oil and perform other routine checks and add star brute to your tank

Im up in Mendocino county Hydro and you should be fine, we are even colder up here and the freezes we get areant really deep freezes and any ice we ever see is melted by 7 or 8 am I know some people that have neglected to drain the block, and have not ever done it and they have been fine. But I would def drain it every time you put it away for the winter

it got down to 22 here one sept night, i put a bulb in the bottom of my bildge to be safe, but i wanted to see what would happen to a gallon of water in a milk jug.... nothing, not even starting to ice over. I think it needs to be in the low twenties for 8-10 hours for anyting to happen.

A quick minimum would be to look for these blue drain plugs and remove them all. This will remove most of the water and give you at least a piece of mind! Get a flash light and really look around for them (down low in at the front of the engine) they can be a little tricky to initially locate. Cheap insurance and will take less than 5 minutes...

I've got an 04 Sanger with a mercruiser (but it's the black scorpion). There are 8 or 9 of those blue plugs and it takes me about 10 minutes to drain everything. I use my wet/dry shop vac to suck up all the water as it's draining so it doesn't end up splashing all over the place. Then I vacuum up the bilge as well.

If you do that, you won't have to worry about the block, manifolds, water pumps, etc., freezing and cracking. The manual suggests (but does not require) filling the block with RV antifreeze but that process is a little more involved.

it got down to 22 here one sept night, i put a bulb in the bottom of my bildge to be safe, but i wanted to see what would happen to a gallon of water in a milk jug.... nothing, not even starting to ice over. I think it needs to be in the low twenties for 8-10 hours for anyting to happen.

That said, if you are nervouse, get it taken care of.

I agree. I live an 1Hr 15Min north of where he is concerened with (colder), and I went out last night and placed a dixie cup on my swim deck and the water didnt freeze...It isnt cold enough long enough here....But i WOULD DRAIN THE BLOCK every winter for sure.

Drain the water, as suggested above. If there is no water there is nothing to freeze. I'm in Canada and have never put antifreeze in a boat. I just make sure it's dry.
A trouble light in the engine compartment will keep things warm but not help with an heater or shower that is not located in the compartment.

I always drain and then run antifreeze through the block. That being said, I'm in a cold climate. Another option that would not require taking out any plugs would be to hook the boat up to a fake a lake and bring it up to temp. Then, just fill a bucket or two with antifreeze and use the fake a lake to suck the antifreeze into the block and manifolds. Run the boat until all you have coming out the exhaust is antifreeze. As noted above, this won't help your heater, at least it doesn't on mine.

I'm a little late, but it's not going to be cold enough for a long enough period to freeze inside the engine. With those day time temps you would need to get down into the low 20s/upper teens for a number of hours to be worried. Does the boat have a heater? Usually it seems as if heater cores freeze way before the engine block.

But you don't only need to worry about the heater and block. What about cracked manifolds, which I had happen to me the first year I owned my old boat because I didn't drain them (oversight). This is in western WA where it rarely if ever gets even down into the upper 20s, and they still froze and cracked. It's not as expensive a repair, but still a PITA when a simple drain will prevent it.

Note my chart on the top of this thread is a snapshot taken from several days worth of data, this one represented the information best. Things might be significantly different if there is wind chill involved.